In medical diagnosis and treatment, it is often desirable to quantitatively measure over time the respiratory air volume. This has conventionally been done by having the patient breathe into a mouthpiece connected to a flow rate measuring device. Flow rate is then integrated to give air volume change. But using the mouthpiece has disadvantages. It is difficult to use for long term patient monitoring, especially for ill, sleeping, or anesthetized patients. Furthermore, it is uncomfortable for the patient, tends to restrict breathing, and is generally inconvenient for the physician or technician to use.
There are qualitative respiration monitors available that do not require a mouthpiece, but none provide an accurate measurement of air volume, and are generally used only to signal an attendant when a patient's breathing activity changes sharply or stops. E.g., Petit U.S. Pat. No. 3,831,586, Hardway et al. U.S. Pat. No. 4,033,332, and Hattes U.S. Pat. No. 3,911,899.
Another way of quantitively measuring lung volume is to measure the change in size of the rib cage and abdomen, as it is known that lung volume is a function of these two dimensions. Magnetometers have been used to make the rib cage and abdomen measurements, as disclosed in Mead, Peterson, Grimby, and Mead, "Pulmonary Ventilation Measured From Body Surface Movements," Science 196: 1383--1384 (1967). Transmitter coils that generate a magnetic field are secured to the front of the patient's chest and abdomen, and receiver coils that are responsive to changes in the magnetic field are secured to the back (or vice versa). Changes in the magnetic field are the result of changes in the separation between transmitter and receiver, i.e., the diameter of the rib cage and abdomen. Further circuitry produces analog voltages proportional to the diameters.
In using these two measurements to predict changes in lung volume, they must first be calibrated, i.e., coefficients that relate them to respiratory air volume must be determined. To accomplish this calibration, prior magnetometer systems (Kono, K. and J. Mead, "Measurement of the Separate Volume Changes in Rib Cage and Abdomen During Breathing," J. Applied Physiology 22: 407-422, 1967) have required that a patient take in a breath through a flow-measuring mouthpiece, and, while holding the breath, shift between two unnatural postures, e.g., an extended abdomen and a retracted abdomen. The patient then breathes in additional air, and again assumes the two postures. Alternatively, the patient could be requested to breathe in two different ways (emphasizing either rib cage or abdominal movements) or in two different postures (e.g., erect or supine). The latter technique is disclosed in Gilbert et al., "Breathing Patterns During CO.sub.2 Inhalation Obtained from Motion of the Chest and Abdomen," Respiration Physiology 13: 238-252 (1977) and Gilbert et al. "Changes in Tidal Volume, Frequency, and Ventilation Induced by Their Measurement," Journal of Applied Physiology, Vol. 33, No. 2, (August 1972). The shift in posture gives two sets of rib cage and abdomen dimensions for the about the same air volume, thereby giving two equations that can be solved for the two unknown coefficients. The unnatural postures required cannot, however, be assumed by many patients, e.g., those who are ill, sleeping, or anesthetized. Furthermore, even when the calibration can be done in this manner, the coefficients obtained are somewhat inaccurate because the air volume is not constant for the two postures due to air compression within the lungs that occurs when the patient assumes the postures. Further error results because of the unnatural spine curvature (i.e., a bending of the trunk) that most patients assume when in the postures.